1. Field of the Invention
The present invention relates to a mask blank for use in preparation of a photomask used in, for example, semiconductor device fabrication, as well as to a protective film for the mask blank and to a method of patterning the mask blank.
2. Description of the Related Art
With an increasing demand on higher density packing of semiconductor devices, micromachining techniques have been developed. To precisely transfer a fine pattern, a highly precise mask pattern is required. Such a mask is generally patterned by application of electron beams. However, as the devices are squeezed into tighter and tighter dimension and the number of traces written by means of electron beams increases, it takes a long time to write traces and thereby the productivity of the mask is decreased.
Alternatively, the mask can be patterned by the use of a chemically amplified resist film. The chemically amplified resist film has been used for patterning of a resist with high sensitivity by using an acid catalytic mechanism, and use thereof in patterning of the mask blank may shorten the imaging or writing time of the mask pattern.
In the chemically amplified resist film, an acid is generated in portions where activating light or radiation such as electron beams or ultraviolet light is applied, and the generated acid decomposes a dissolution inhibitor which inhibits the resist film from dissolving in bases, and thus the resist film become soluble in bases.
Such chemically amplified resist films are commercially available, for example, from FUJIFILM OLIN Co., Ltd. under the trade name of FEP-171 and from Tokyo Ohka Kogyo Co., Ltd. under the trade name of EP-009.
FIGS. 6A through 6E are views illustrating a process for fabrication of a photomask by patterning such a conventional chemically amplified resist film. The figures show a substrate 1, a light-block film 2, a chemically amplified resist film 3, and activating light or radiation 4 such as electron beams or ultraviolet light.
The quartz substrate 1 carries the light-block film 2 formed thereon. The light-block film 2 has a film thickness of 50 to 300 nm and is made of, for example, chromium (Cr) or MoSiON. Initially, the chemically amplified resist is applied onto the light-block film 2 to a film thickness of 100 to 500 nm and thereby yields the chemically amplified resist film 3. The substrate 1 is then subjected to first heating treatment (prebaking) and thereby yields a mask blank (FIG. 6A).
The mask blank is then irradiated with electron beams in vacuo at an accelerating voltage of 10 to 50 kV or with ultraviolet rays in the air (FIG. 6B). In this procedure, the mask blank is patterned by writing desired traces one by one by means of electron beams or ultraviolet rays.
An acid is generated only in exposed portions of the chemically amplified resist film 3 which are exposed to activating light or radiation. When the substrate 1 is subjected to second heating treatment after exposure (post exposure baking, PEB), the acid in the chemically amplified resist film 3 acts as a catalyst, decomposes most of a dissolution inhibitor and makes the exposed portions soluble in bases to thereby pattern the chemically amplified resist film 3.
Specifically, the exposed mask blank is developed with a developing solution to thereby pattern the chemically amplified resist film 3 on the light-block film 2 (FIG. 6C).
When the light-block film 2 is made of Cr, it is subjected to magnetic field applied reactive ion etching using a gaseous mixture of chlorine or hydrogen chloride with oxygen at an applied magnetic field strength of 50 G for 10 minutes to thereby transfer the written traces of the resist to the Cr film (FIG. 6D).
When the light-block film 2 is made of MoSiON, it is subjected to magnetic field applied reactive ion etching using a gaseous mixture of carbon tetrafluoride and oxygen at an applied magnetic field strength of 50 G for 5 minutes to thereby transfer the written traces of the resist to the MoSiON film as in the Cr film (FIG. 6D).
The patterned resist, after etching, is stripped using an oxygen ashing system or a resist stripper solution and thereby yields a photomask (FIG. 6E).
The photomask is fabricated according to the above process. However, with downsizing of semiconductor devices and with an increasing number of traces to be written, it takes about 6 to 40 hours and, on average, about 12 hours to write traces in one mask blank (FIG. 6B).
The highly sensitive chemically amplified resist is used to shorten the time to write or image traces of the mask, but still it takes several hours to image the traces and thereby induces changes in finished dimension.
In addition, when the mask blank is stored over a long period of time, the finished dimension also changes.
These problems are probably induced for the following reasons. Specifically, a chemically amplified resist for use in the mask blank is configured in such a manner that it enhances decomposition reactions by action of a trace amount of an acid catalyst in order to achieve high sensitivity. However, a basic substance migrates into the mask blank during exposure or storage of the mask blank, neutralizes the acid generated upon irradiation of activating light or radiation and thereby significantly deteriorates decomposition reactions by action of the acid catalyst. Thus, the pattern dimension is significantly changed, an insolubilized surface layer is formed, or the traces are not resolved.
Specifically, when the resist is exposed and patterned in vacuo, the acid evaporates and significantly deteriorates the decomposition reaction by catalysis of the acid catalyst to thereby significantly affect the pattern dimension.
To avoid these problems, a protective film is formed on the chemically amplified resist film. For example, Japanese Patent Laid-Open Nos. 5-216244, 6-95397 and 7-209875 each disclose a technique in which a film of a water-soluble resin or a neutral organic (water-insoluble) resin is formed on the chemically amplified resist film 3 for mask fabrication to thereby prevent diffusion of such a basic substance into the resist film.
However, the formation of such protective film made of a water-soluble resin or neutral organic resin only enables stable patterning for a very short time.
This is because, even when the protective film is formed on the chemically amplified resist film of the mask blank, basic substances in the atmospheric air permeate the protective films during exposure or storage of the mask blank and then migrate and diffuse into the chemically amplified resist film.
In addition, the use of the protective film does not sufficiently prevent the acid from evaporating during exposure of the mask blank in vacuo.
Specifically, the conventional protective films can only effectively act for a short time and do not enable highly precise and stable patterning of the mask blank which must be stable over a long period of time.